How can i permanently delete videos on my galaxy samsung


You can format the SD card and the phone has a function for that in the settings, if the photos are in the phones internal memory the only way to delete those are to reset the phone to factory condition. Once data and other photos are written over the deleted ones they become permanently deleted.
Another way to erase the SD card is to put the SD card in a card reader and with using the computer and an application like Erasure you can use a high powered method of permanently erasing.